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    label标签用于绑定一个表单元素，当点击标签内文本时，浏览器就会自动将焦点转到对应的表单元素上，增加用户体验
    label标签的for属性应当与id属性相同<br>
    我是：<input type="radio" name="sex" id="nan"><label for="nan">男</label> 
    <input type="radio" name="sex" id="nv"><label for="nv">女</label>
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